The optimum separation membrane for use in any gas separation combines high selectivity with high flux. Thus the membrane industry has engaged in an ongoing quest for membranes with improved flux/selectivity performance.
In recent years, some polymer materials with unusually high permeabilities have been synthesized. Perhaps the best known of these, and representative of the class, is polytrimethylsilylpropyne (PTMSP), a polymer synthesized by T. Masuda et al. in Japan. Although PTMSP is glassy, up to at least about 200.degree. C., it exhibits an oxygen permeability of 10,000 Barrer or above, more than 15 times higher than that of silicone rubber, previously the most permeable polymer known. The selectivity for oxygen/nitrogen, however, is low (1.4-1.8). The high permeability appears to be associated with an unusually high free-volume within the polymer material, and has been confirmed with many examples of pure gases and vapors, including oxygen, nitrogen, hydrogen, helium, methane, ethane, propane, butane and higher hydrocarbons, sulfur hexafluoride and carbon dioxide. These pure-gas data suggest that PTMSP will exhibit poor selectivity for most gas separations.
Thus the material was characterized, at least initially, as of great academic interest, because of its extraordinary permeability, but exhibiting selectivities too low for commercial use. Later, however, it was found that the measured actual mixed-gas selectivity for more condensable organic compounds over less condensable organic compounds or inorganic compounds is dramatically better than the calculation from the pure gas permeabilities would suggest, and that useful processes for separating condensable components from gas streams using glassy high-free-volume polymers are possible. Such processes are disclosed in U.S. Pat. No. 5,281,255, which is incorporated herein by reference in its entirety.
An issue that has concerned all workers with these glassy, high-free-volume materials is stability. Many reports that the permeation properties of PTMSP appear to be unstable over time have been published. For example, Masuda et al. found that the oxygen permeability fell to about 1% of its original value when the membrane was left at room temperature for several months. Odani et al. found that the butane permeability fell by about two orders of magnitude when the material was stored under vacuum at 30.degree. C. for 100 days.
The consensus of opinion in the art has been that the loss in permeability arises from physical aging, producing a gradual loss of free volume, and/or sorption of organic vapors into the high free volume of the polymer. For example, a paper by T. Nakagawa et al. ("Polyacetylene derivatives as membranes for gas separation", Gas Separation and Purification, Vol. 2, pages 3-8, 1988) states that "the reason for unstable gas permeability is the adsorption of volatile materials existing in the atmosphere."
As far as separation of non-condensable gases is concerned, the loss of permeability over time is accompanied by an increase in selectivity. On the other hand, for separation of condensable from non-condensable components, the aging effect causes not only a loss in permeability, but, which is more important, a serious loss of selectivity for the condensable over the non-condensable component.